'antimystically' originates from Modern English, formed from the prefix 'anti-' (from Greek 'anti' meaning 'against') + 'mystical' (from 'mystic' + adjectival suffix '-al'), with the adverbial suffix '-ly' added.

'mystic' comes from Greek 'mystikos' (meaning 'secret, pertaining to mysteries') via Latin 'mysticus' and Old French 'mystique'; English 'mystic' and 'mystical' developed in Middle English, and the combining prefix 'anti-' was attached to form 'antimystical', later producing the adverb 'antimystically'.

Initially, roots referred to 'mysteries' or 'secret rites' (Greek 'mystikos'); over time 'mystical' came to mean 'relating to mysticism or spiritual mystery', and 'anti-' gave the sense 'against', so the compound now means 'against or rejecting mysticism', expressed adverbially as 'in an antimystical way'.